High‐resolution submillimeter‐wave radiometry of supersonic flow

The recent development of a high‐resolution submillimeter‐wave heterodyne radiometer has made possible the first measurements of H2O molecule rotational line excitation temperatures and detailed profiles in supersonic flow. Absorption signals were measured across the flow for the 211←202 (752 GHz) para‐H2O rotational transition against a hot background. These signals decrease downstream owing to the volume expansion of the gas away from the sonic nozzle exit in the high‐vacuum chamber. Radiative transfer calculations based on the large‐velocity‐gradient approximation and multilevel statistical equilibrium agree with these results and with the measured spectral line shapes. The data reveal nearly isentropic gas expansion and cooling. These studies have shown that submillimeter‐wave heterodyne radiometry can be useful for remote sensing of supersonic flow with low mass flux, provided the signal transmission is through a dry or thin atmosphere.